A fundamental difficulty with such display devices is that of superimposing the normal beam path from the surroundings with the beam path of the light of the image generated by the image-generating module and conveying it to the eye of the user who is wearing the display device on his head. The beam path of the light of the generated image considered on its own represents an eyepiece, which projects the image information of the image-generating module into the eye. Now if the eyepiece is designed such that an image field that is as large as possible is obtained with a large exit pupil at the same time, this requires the use of deflecting elements in the form of mirrors or prisms. These additional deflecting elements increase the weight and volume of the complete display device, which for example significantly limits wearing comfort.
The curvature of the front and back of the spectacle lens leads to the further difficulty that large image errors arise during conduction in the optical channel, which must be compensated for.